batman-adv: fix panic during interface removal

Reference counting is used to ensure that
batadv_hardif_neigh_node and batadv_hard_iface
are not freed before/during
batadv_v_elp_throughput_metric_update work is
finished.

But there isn't a guarantee that the hard if will
remain associated with a soft interface up until
the work is finished.

This fixes a crash triggered by reboot that looks
like this:

Call trace:
 batadv_v_mesh_free+0xd0/0x4dc [batman_adv]
 batadv_v_elp_throughput_metric_update+0x1c/0xa4
 process_one_work+0x178/0x398
 worker_thread+0x2e8/0x4d0
 kthread+0xd8/0xdc
 ret_from_fork+0x10/0x20

(the batadv_v_mesh_free call is misleading,
and does not actually happen)

I was able to make the issue happen more reliably
by changing hardif_neigh->bat_v.metric_work work
to be delayed work. This allowed me to track down
and confirm the fix.

Cc: stable@vger.kernel.org
Fixes: c833484 ("batman-adv: ELP - compute the metric based on the estimated throughput")
Signed-off-by: Andy Strohman <andrew@andrewstrohman.com>
[sven@narfation.org: prevent entering batadv_v_elp_get_throughput without
 soft_iface]
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>

Author: andrewstrohman

net/batman-adv/bat_v_elp.c

@@ -66,12 +66,19 @@ static void batadv_v_elp_start_timer(struct batadv_hard_iface *hard_iface)
 static u32 batadv_v_elp_get_throughput(struct batadv_hardif_neigh_node *neigh)
 {
 	struct batadv_hard_iface *hard_iface = neigh->if_incoming;
+	struct net_device *soft_iface = hard_iface->soft_iface;
 	struct ethtool_link_ksettings link_settings;
 	struct net_device *real_netdev;
 	struct station_info sinfo;
 	u32 throughput;
 	int ret;
 
+	/* don't query throughput when no longer associated with any
+	 * batman-adv interface
+	 */
+	if (!soft_iface)
+		return BATADV_THROUGHPUT_DEFAULT_VALUE;
+
 	/* if the user specified a customised value for this interface, then
 	 * return it directly
 	 */
@@ -141,7 +148,7 @@ static u32 batadv_v_elp_get_throughput(struct batadv_hardif_neigh_node *neigh)
 
 default_throughput:
 	if (!(hard_iface->bat_v.flags & BATADV_WARNING_DEFAULT)) {
-		batadv_info(hard_iface->soft_iface,
+		batadv_info(soft_iface,
 			    "WiFi driver or ethtool info does not provide information about link speeds on interface %s, therefore defaulting to hardcoded throughput values of %u.%1u Mbps. Consider overriding the throughput manually or checking your driver.\n",
 			    hard_iface->net_dev->name,
 			    BATADV_THROUGHPUT_DEFAULT_VALUE / 10,